Numerous polymer-based medical devices have been developed for implantation or insertion into the body. For example, various state of the art medical devices consist of a medical device substrate with a polymeric coating that serves as a reservoir for one or more therapeutic agents. Specific examples include drug eluting coronary stents, commercially available from Boston Scientific Corp. (TAXUS), Johnson & Johnson (CYPHER) and others, which have become the standard of care for maintaining vessel patency after balloon angioplasty. These products are based on metallic balloon expandable stents with polymeric coatings that release antiproliferative drugs at a controlled rate and total dose effective to inhibit the smooth muscle proliferation that is associated with restenosis (vessel reclosure).
Various types of polymeric materials have been used as drug-releasing reservoirs, including, for example, homopolymers such as poly(n-butyl methacrylate) and copolymers such as poly(ethylene-co-vinyl acetate), copolymers containing phosphoryl choline acrylate, and copolymers such as poly(isobutylene-co-styrene), for example, poly(styrene-b-isobutylene-b-styrene) triblock copolymers (SIBS), which are described, for instance, in U.S. Pat. No. 6,545,097 to Pinchuk et al. In addition to their utility as drug delivery reservoirs, SIBS copolymers have proven valuable for a variety of reasons, including their excellent biocompatibility, elasticity, strength, and processability. The latter characteristics are due, at least in part, to the fact that SIBS copolymers are thermoplastic elastomers. Thermoplastic elastomers are elastomeric (i.e., reversibly deformable) polymers that form physical crosslinks which can be reversed, for example, by dissolving or melting the polymer. SIBS triblock copolymers have an elastomeric low glass transition temperature (Tg) midblock and hard elevated Tg endblocks. As with many block copolymers, SIBS tends to phase separate, with the elastomeric blocks aggregating to form elastomeric phase domains and the hard blocks aggregating to form hard phase domains. It has been hypothesized that, because each elastomeric block has a hard block at each end, and because different hard blocks within the same triblock copolymer are capable of occupying two different or separate hard phase domains, the hard phase domains become physically crosslinked to one another via the soft blocks. Another embodiment of a phase separated thermoplastic elastomer consists of endblocks of polymethylmethacrylate and a midblock of polybutylacrylate (MBAM). The resulting desirable properties result from similar phase separation of the methacrylate hard blocks into hard block domains and the butylacrylate soft blocks into a soft block domain.
In a current process for forming TAXUS products, the outer surface of a stainless steel coronary stent is sprayed with a solution that contains solvent, paclitaxel and SIBS. The solution is sprayed on the outside of the stent, and to some degree, through the stent struts. The stent is ultimately encapsulated with the polymeric coating due to a combination of outside spraying and through-strut spraying combined with flow of the solution around the stent struts. The net result is that the spray process results in a conformal coating. The result of such a process is schematically illustrated, for example, in FIGS. 1A and 1B. FIG. 1A shows a stent 100 which contains a number of interconnected struts 100s. FIG. 1B is a cross-section taken along line b-b of strut 100s of stent 100 of FIG. 1A, and shows a stainless steel stent substrate 110 and a paclitaxel-containing polymeric coating 120, which encapsulates the substrate 110. The coating has relatively poor adhesion to the stent substrate surface. However, it is nonetheless well-secured to the stent substrate as a result of the encapsulation that occurs (and the inherent cohesive strength of SIBS).
While it is desirable to provide the abluminal surface of the stent with a polymeric coating that that is capable of releasing an antiproliferative drug to combat restenosis, such a drug may not be equally desirable on the luminal surface of the stent and, in fact, may even be detrimental to the extent that it may retard or otherwise interfere with the growth of healthy endothelial cells on the luminal surface of the stent. Moreover, the presence of a polymeric layer on the luminal surface is not needed for purposes of promoting biocompatibility, as various stent substrate materials, including stainless steel, are known to support endothelial cell growth. In addition, it may be desirable to minimize the total polymer content in a drug coated stent in order to minimize any potential undesirable biological responses to the polymer.